Alejandro J. Bisigato

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.

Spatial sex segregation in the dioecious grass Poa ligularis in northern Patagonia: the role of environmental patchiness

We examined the effect of environmental patchiness on the spatial segregation of the sexes in the dioecious anemophilus grass Poa ligularis. Because the species is sensitive to grazing, a better understanding of environmental factors that control its spatial distribution and abundance could improve conservation efforts. We hypothesized that (i) males and females are spatially segregated in the microenvironments created by plant patches as the result of sexual specialization in habitat and/or resources use, (ii) sexual specialization is related to different tolerance to competition and reproductive costs of males and females, and (iii) changes in patch structure affect the microenvironment and the intensity of spatial segregation of the sexes. We analyzed the spatial distribution of sexes at three sites with different plant and micro-environmental patchiness and performed a controlled competition experiment with different substitution of males and females. Our results showed that large plant patches created larger sheltered soil fertility islands than small patches. As patch size and their area of influence increased, the density and the spatial segregation of the sexes of P. ligularis also increased, resulting in biased habitat-specific sex ratios. In accordance with their higher reproductive costs, females were more frequent in sheltered (low air evaporative demand) and nitrogen-rich areas inside patch perimeters than males. Females were also better able to tolerate inter-sexual competition than males. In contrast, males tolerated low nitrogen concentration in soil and low sheltering, probably gaining advantage in pollen dispersal. Inter- and intra-sexual competition, however, affected the reproductive output of both sexes. From the point of view of conservation, environmental patchiness is important to the status of P. ligularis populations. The reduction of patch size limits the available microsites, biases the sex ratio towards males inside patches, increases inter- and intra-sexual competition, and it might be expected to decrease overall seed and pollen production and consequently potential recruitment.

Seedling emergence and survival in contrasting soil microsites in Patagonian Monte shrubland

The potential of two perennial species (Larrea divaricata and Stipa tenuis) to colonize different soil microsites was analyzed in the Patagonian Monte shrubland. We hypo- thesize that the short-lived grass S. tenuis is more able to colonize the soil of microsites beneath vegetation patches where N-fertility is higher than those in bare soil, while the long-lived shrub L. divaricata colonizes different soil microsites irrespective of their N fertility. A greenhouse experiment was carried out to evaluate the emergence and survival of both species in different soil microsites at different water, inorganic N and seed densities. In all cases soil microsites were seed limited since the addition of viable seeds increased seedling emergence. Both species showed, however, different abilities to emerge and survive in different soil microsites. Microsites of bare soil were more favourable for seedling emergence and survival ofL. divaricata than those beneath vegetation patches, independent of their water status. This ability of L. divaricata can not be explained on the basis of increased water or N availability, but probably because of lower salt content of bare soil. The addition of inorganic N reduced the survival of L. divaricata in both microsites but increased individual plant performance. The emergence and survival of S. tenuis was not different in both types of soil microsites but the addition of inorganic N increased seedling emergence and plant biomass. According to these results, emergence and plant performance of S. tenuis may be promoted during humid years by increased concentration of inorganic N. Since N mineralization occurs at a higher rate in soil microsites beneath vegetation patches than in those of bare soil, higher plant performance and probably establishment of S. tenuis is to be expected. These results are consistent with an existing conceptual model of plant dynam- ics under various grazing intensities in the Patagonian Monte shrubland based on previous field observations.

Temporal and micro-spatial patterning of seedling establishment. Consequences for patch dynamics in the southern Monte, Argentina

We compared the temporal and micro-spatial patterning of seedling emergence and establishment of two cohorts of perennial grasses and shrubs in the southern Monte, Argentina. Samplings were carried out in two contrasting areas (grazed and non-grazed) during four years. We found lower densities of emerged and established seedlings of perennial grasses in the grazed relative to the non-grazed area. Conversely, emerged seedlings of shrubs did not vary between the grazed and the non-grazed area and densities of established shrub seedlings were higher in the grazed than in the non-grazed area. We only found differences between cohorts in seedling emergence of perennial grasses. These differences could be associated with the amount of precipitation in the year previous to the emergence. Both in the grazed and non-grazed area, seedlings of perennial grasses were concentrated at the periphery of plant patches. We defined a plant patch as a discrete unit of the spatial pattern of vegetation surrounded by, at least, 20 cm of bare soil from the nearest neighbour patch. Emergence in perennial grasses was more frequent at the southern/western patch-periphery than at other patch-periphery locations. Established seedlings of perennial grasses, however, were homogeneously distributed throughout patch periphery. Emergence in shrubs was more frequent at the centre and periphery of patches than at inter-patch microsites. In contrast, established seedlings of shrubs were homogeneously distributed among microsites. Our results suggests that differential seedling establishment between life forms is the outcome of complex biotic and abiotic interactions and feedbacks at the patch level between seedlings and established plants. Both life forms appear to have a different role in the origin, dynamics, and maintenance of spotting vegetation. Because of the ability to establish both at inter-patch and patch microsites, shrubs could be identified as colonizers or initiators of small plant patches in bare soil or they may contribute to increase the cover and size of pre-existing plant patches. Both processes would be promoted in grazed areas. Due to the ability to establish at patch peripheries, perennial grasses would contribute to the isodiametric growth of pre-existing patches, preferentially in non-grazed areas.

Modeling and Measurement of Structural Changes at a Landscape Scale in Dryland Areas

We present a technique to quantify and model the intensity of structural changes produced by management of dry grazing lands at a landscape scale. The technique is illustrated with the analysis of digitized black–white (b/w) imagery and an application to the study of changes induced by grazing gradients. Structural changes in patchy vegetation canopies were studied in the Patagonian Monte (Chubut, Argentina) at two resolution scales by means of linear transects in the field (50 m) and others drawn on aerial b/w photographs (2–5 km) of grazed paddocks. Spatial series of plant cover values along transects in the field and on photographs were analyzed with standard techniques of spectral analyses, including auto-correlation spectra and Fourier transforms. In order to test the internal consistency of the techniques used, synthetic plant canopies with patches of varying cover and size were generated by means of a stochastic model of plant growth under different stocking rates or after varying periods of recovery. The behavior of the simulation model is consistent with the observed dynamics of plant canopies in semiarid environments. There is a consistent relation between the number and geometric properties of plant patches (patch number, patch size, patch connectivity) and the signal/noise ratios of the Fourier decomposition describing plant density data. Signal/noise ratios corresponding to plant cover data in paddocks with different grazing treatment are consistent with the assumptions derived from modeled canopies, as well as those estimated from optical density of b/w aerial photographs of paddocks. We tested the hypotheses that patch arrangements as quantified by the signal/noise ratios vary in accordance with grazing gradients in paddocks with a permanent corner-located watering point. The use of digitized b/w images allows inspecting permanent changes over time periods when other types of images were not yet available.

Seedling recruitment of perennial grasses in degraded areas of the Patagonian Monte

Abstract The recruitment of perennial grass seedlings in degraded areas of the Patagonian Monte was analyzed. Recolonization of large bare-soil areas by dwarf shrubs or perennial grasses was hypothesized to create favorable microsites for grass seedling recruitment. Under natural field conditions, soil moisture (0–20 cm), root biomass (0–20 cm), the soil seed bank of perennial grasses, and density of perennial grass seedlings in the center of large bare soil areas and in microsites neighboring isolated plants of perennial grasses and dwarf shrubs were assessed. A manipulative experiment was conducted to evaluate seedling recruitment at the 3 microsites with identical density of seed addition of perennial grasses, under situations of root exclusion, and water addition. Under natural field conditions, root biomass did not differ among microsites. Soil moisture, seed density, and seedling density of perennial grasses were higher near perennial grasses than in bare soil or next to dwarf shrubs. Recruitment of perennial grass seedlings did not differ among microsites when the density of seeds did not vary among them or roots of the established plants were excluded. Water increased perennial grass seedling recruitment at all microsites. In degraded areas of the Patagonian Monte the spatial distribution of the soil seed bank followed by water availability are the main limiting factors of seedling recruitment of perennial grasses. Managerial practices oriented to maintain and increase the soil seed bank of perennial grasses, such as grazing exclusion during the grass reproductive period, might contribute to promote the re-establishment of perennial grasses.

Estimates of dryland degradation in Argentina with Fourier signatures from low-altitude monochromatic images with high spatial resolution

Most world drylands are used as graziny lands and undergo degradation of their vegetation cover. The plant cover is typically structured in patchy arrangements, inducing fertility islands critical to maintenance of ecosystem properties. The characteristics of patch structure (size of patches, connectivity-continuity of patch units, etc.) are indicators of the degree of dryland deterioration. We characterized changes in patch structure induced by sheep grazing at a landscape scale using monochromatic low-altitude imagery digitized to a spatial resolution of about 1 m with standard techniques of harmonic analysis applied to develop Fourier signatures. The signatures developed on image line transects were tested with ground samples and mathematical models of plant cover in several dryland fields where spatial deterioration gradients existed. The sensitivity and errors associated to long-wave noise introduced by the geometry of the camera-field-sun spatial arrangement and to high frequency noise introduced by the digitizing process were evaluated by applying suitable filters in the frequency domain. Fourier signatures developed on monochromatic low-altitude imagery proved to be indicative of changes in the patching arrangements of plant cover. We concluded that adequately filtered, high spatial resolution monochromatic images can be used to evaluate the degree of deterioration of dryland landscapes through the computation of selected Fourier signatures in their frequency domain. At comparable cost, aerial photography allows inspecting the landscape at higher spatial resolutions than those attainable with satellite imagery. Also, aerial photos of many areas are available for earlier dates than images from remote sensors, which would allow better inspection of long-term ecosystem changes.

Interacting effects of soil degradation and precipitation on plant productivity in NE Patagonia, Argentina

ABSTRACT Our objective was to examine the effects of inter-annual variation of precipitation on productivity of two dominant species (Chuquiraga avellanedae, an evergreen shrub, and Nassella tenuis, a perennial grass) in two communities of contrasting soil degradation: a herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Data were collected during two consecutive years with different annual precipitation. Aboveground productivity was determined nondestructively using a double sampling approach. The number of inflorescences per plant was recorded too. Perennial grass productivity was lower in SS than in HSS in both years, while shrub productivity was lower in SS only during the year of below average precipitation. With rising precipitation the perennial grass increased the number of inflorescences while the evergreen shrub augmented vegetative biomass. In summary, the effects of precipitation on plant productivity are community dependent; abiotic factors, such as superficial and sub-superficial soil characteristics, and biotic factors, such as leaf area index (LAI) or tussock sizes, may interact to influence the responses of species to precipitation. Our results suggest that if precipitation increased, this would favor the dominance of shrubs over grasses.

Leaf traits, water stress, and insect herbivory: Is food selection a hierarchical process?

Abstract Plant water stress can affect selectivity by insect herbivores. Numerous studies have shown greater insect preference for water-stressed plants, but others have reported the opposite response. We evaluated leaf consumption by adults of Nyctelia circumundata (a chewing insect) in leaves of Larrea divaricata and Prosopis alpataco. Three bioassays (two-way choice tests) were performed: two intra-specific comparisons between well-watered (+W) and water-stressed (−W) leaves of each species and one inter-specific comparison between leaves of the two species. Leaf biomass was reduced by water stress in both species. Nitrogen concentration in leaves (N) was reduced by drought in P. alpataco. In contrast, total phenolics and specific leaf area (SLA) did not differ between treatments within species. Nyctelia circumundata did not show preference by any water supply regimes in intra-specific comparisons. In contrast, in inter-specific choice tests, it showed a marked preference for P. alpataco, which is the species with the highest nitrogen concentration and lowest total phenolics concentration. In intra-specific comparisons, maximum leaf consumption was inversely related to SLA in both species. Furthermore, in P. alpataco, N concentration was positively related to maximum leaf consumption and negatively related to leaf water content (LWC). In contrast, in inter-specific comparisons, total phenolics was negatively related to maximum leaf consumption, while N concentration exhibited the opposite trend. These results suggest that food selection is a hierarchical process where chemical attributes (i.e., total phenolics and N) are taken into account for species selection, and physical attributes (i.e., SLA and LWC) for choosing individuals inside species.

Plant production along a grazing gradient in a semiarid Patagonian rangeland, Argentina

Patagonian rangelands have been grazed by sheep since early twentieth century. However, there is still a degree of uncertainty regarding how production of grass and shrub species changes along a grazing gradient. The study was undertaken in Northeastern Patagonia, Argentina. The characteristic vegetation in the area is a mosaic of herbaceous steppe with shrubs (HSS) and shrub steppe (SS). Grazing intensity was estimated through sheep paths density. Individual plant production and plant density were used to determine grass and shrub production per unit area, in both plant communities over three years. Community production was obtained as the sum of grass and shrub components. Differences were explored in shrub and grass production among communities and years, and linear regressions between sheep paths density and the proportion of each plant community along the transect were performed. Mean community production was lower in SS than HSS; shrubs did not compensate for the decline in grass production in spite of the increase in shrub density. SS presented the highest community production in the most humid year, while HSS production peaks in the year of average precipitation. We found that as the number of sheep paths increases (i.e., higher grazing intensities), the SS community replaces HSS, resulting in a reduction of forage for sheep. The results showed that changes in vegetation structure as a result of grazing strongly influenced above-ground production. Results also indicate that the response of vegetation to changes in annual precipitation is community specific.